As a substrate that develops color by being oxidized, a methylene blue-based color former such as, for example, 10-(carboxymethylaminocarbonyl)-3,7-bis(dimethylamino)phenothiazine has been known. Methylene blue that is a chromophore in the methylene blue-based color former hardly fades in color, thereby allowing highly sensitive detection to be performed, and thus it is expected that the methylene blue-based color former will be used in various analyses.
Such a color former can be used, for example, in the case of determining the amount of an oxidizing substance such as hydrogen peroxide generated using an oxidoreductase by causing a reaction between the color former and the oxidizing substance and then measuring the amount of a color developed by measuring an absorbance. When a color former is to be used in such an enzyme reaction, generally, a solution prepared by dissolving the color former in water is used as a liquid reagent.
However, a methylene blue-based color former as described above has presented a problem of being so unstable as to develop color spontaneously when in a solvent such as water. Because of this, the use of a color former that has been stored in a liquid state may cause an increase in background absorbance in a measurement of an absorbance, thus degrading the accuracy of the measurement. In particular, although it is advantageous that, as described above, methylene blue that is a chromophore in a methylene blue-based color former has a property of hardly fading in color and thus achieves high sensitivity, methylene blue has presented a problem that part of the chromophore (methylene blue) that has been freed by natural oxidation also hardly fades in color compared with other chromophores, thus exerting an influence that hardly can be avoided. For example, when stored in an aqueous solution state, 10-(carboxymethylaminocarbonyl)-3,7-bis (dimethylamino)phenothiazine develops enough color spontaneously to exert an influence on a measurement within about one day, and deteriorates to such an extent that it hardly can be used as a reagent for an analysis any more within about three days (see Patent Document 1 or U.S. Pat. No. 4,916,058 corresponding thereto).
In order to prevent such an influence of spontaneous color development, it is required to prepare a liquid reagent every time a measurement is performed. This, however, leads to a complicated operation and also to a cost increase.    Patent Document 1: JP 4(1992)-27839 B